I can't open Blender 2.8 files after I save them. Because it tries to open with 2.79
You don't need to delete anything.
Just open blender 2.8, in the top menu select File > Open and open the file created in 2.8

If you want to permanently open all *.blend files with 2.8 you would need to modify your operating system's default applications (do a search online for that. Customizing the operating system is not a topic for this site).